Magnetic tunnel junctions (MTJ) form promising candidates for nonvolatile memory storage cells to enable a dense, fast, nonvolatile magnetic random access memory (MRAM). The magnetic tunnel junction comprises at least two ferromagnetic layers separated by a thin insulating layer. The conductance of the device depends on the relative magnetic orientation of the magnetic moments of the ferromagnetic layers. The lateral size of the MTJ storage cell must be of sub-micron dimensions to be competitive with today's DRAM memories with 10-100 Mbit capacities. Moreover, the lateral size of the MTJ storage cell will need to be further reduced as memory capacities further increase in the future.
The required small sample size of the MTJ storage cell leads to several problems. First, as the lateral dimensions of the cells are reduced, the volume of each of the magnetic layers in the MTJ device are also reduced, which leads to the possibility of “super-paramagnetic” behavior, i.e., thermal fluctuations can cause the magnetic moment of a magnetic entity to spontaneously rotate or flip. Even though this could be addressed by increasing crystalline or shape anisotropy of the magnetic entity, such increase is not practical, as it would require increasingly higher magnetic fields, and thus currents, to controllably switch the magnetic state of the cell. Secondly, with increasing density of the cells, the distance between cells is reduced, leading to the increase of the magnetic field at a cell location produced by the magnetization of the neighboring cells. Thus, the magnetic switching field of a given MTJ cell will depend on the magnetic state of its neighboring cells, leading to either higher margin of the write operation of the memory array, or to spontaneous switching of the cell due to the state of its neighbors. Unless these magnetostatic interactions can be mitigated they will eventually limit the smallest size attainable by the MTJ cells and the highest density of the MTJ MRAM.
In order to overcome these limitation and make a more stable MRAM device, MTJs have been constructed such that either or both of the free and pinned ferromagnetic layers are constructed each as a pair of antiparallel coupled ferromagnetic layers separated by a non-magnetic spacer layer. Such devices are described in IBM U.S. Pat. Nos. 6,166,948 and 5,966,012. An example of such a prior art MTJ is described with reference to FIG. 1. The prior art MTJ 2 is sandwiched between first and second electrically conductive lines 3, 5 which are arranged perpendicular to one another with line 5 extending out of the plane of the page. The MTJ 2 is disposed entirely between the lines 3 and 5, and electrically connects them with one another. The resistance, and associated memory state, of the MTJ 2 is determined by applying a voltage across the MTJ between the lines 3 and 5. The MTJ 2 includes a free ferromagnetic layer 4 and a pinned ferromagnetic layer 6. The free layer 4 includes first and second ferromagnetic layers 8, 10 which sandwich an antiferromagnetic coupling layer 12 therebetween. The antiferromagnetic coupling layer can be constructed of Ru and is of such a thickness as to antiparallel couple the first and second ferromagnetic layers 8, 10 with one another. Similarly, the pinned layer 6 is constructed of first and second ferromagnetic layers 14, 16, which are antiparallel coupled across a Ru coupling layer 18. The free and pinned layers 4, 6 are separated by a tunnel barrier layer 20 such as Al2O3. The Pinned layer 6 is formed upon an anitferromagnetic (AFM) material 22 such as PtMn. Strong exchange coupling between the AFM layer 22 and the second layer 16 of the pinned layer 6 keeps the second layer 16 strongly pinned along a predetermined direction, preferably along its easy axis of magnetization. The antiferromagnetic coupling across the Ru coupling layer 18, keeps the first layer 14 strongly pinned antiparallel to the second ferromagnetic pinned layer 16 as indicated by arrows 24. The ferromagnetic layers 8, 10 have a magnetic anitsotropy that tends to keep their magnetizations aligned along an axis that is parallel with the magnetizations of the pinned layer 6 as indicated by arrows 26.
With continued reference to FIG. 1, it will be appreciated by those skilled in the art that, with the magnetizations of the free and pinned layers 4, 6 aligned as shown, the MTJ will be in a high resistance state. In order to put the MTJ into a low resistance state, and thereby change its memory state, an electrical current is passed through a conductive line 5 which runs along a direction perpendicular to the directions of magnetization of the free and pinned layers 4, 6. A current directed out of the plane of the page as indicated by arrow head 30 will induce a magnetic field thereabout according to the right hand rule as indicated by arrow 32.
The antiparallel coupling of the free layer promotes stability of the free layer making it less susceptible to unintentional switching due to temperature or extraneous magnetic fields. In addition, the antiparallel coupling of the pinned and free layers 4, 6 reduces undesirable magnetostatic coupling between the pinned and free layers 4, 6 which would otherwise cause the free layer to be biased toward one of its two possible magnetic states. Unfortunately, this prior art MTJ array requires a relatively strong magnetic field to switch the magnetic state of the free layer. This requires a large electrical current to be passed through the conductive lines 5 and 3 which increases power consumption to unacceptably high levels. Also, the high field necessary to switch the free layer increases the risk of affecting adjacent MTJ cells, requiring undesirably high spacing between adjacent cells.
Therefore, there is a need for a MTJ array having MTJ cells which can be efficiently switched while also being magnetically stable. Such a MTJ array would preferably be minimally affected by extraneous magnetic fields and temperature fluctuations, while requiring a minimum of energy to switch from one memory state to another.